The invention described and claimed herein has application to accelerators used to produce charged particle beams, primarily electron beam accelerators. While the present invention is described herein primarily with reference to electron beam accelerators, the invention also has application to accelerators designed to produce beams of protons or other charged particles.
All of the references cited herein are hereby incorporated by reference.
In most charged particle accelerators there is a need to determine the size, position, cross-sectional shape, and other characteristics of the beam of charged particles produced by the accelerator, usually at various points along the path of the beam as it is accelerated along an evacuated beam tube. Such a determination is necessary in order to enable appropriate adjustments to be made to the structures and operating parameters of the accelerator, for the purpose of optimizing the size, shape, position and other characteristics of the beam.
Since the particles constituting the beam are electrically charged, they interact with an electrically resonant cavity interposed along the beam path. This interaction provides the basis for accelerating the particles, by applying a radio frequency signal to the cavity from an external source. In this regard, a particle beam accelerator will typically have a substantial number of resonant cavities, up to hundreds or thousands, positioned in sequence along the beam path. The purpose and function of each such cavity is to accelerate the particles as they pass through the cavity. At each stage additional energy is imparted to the charged particles, to the extent that in an electron beam accelerator the electrons are typically accelerated to a velocity that is a substantial fraction of the speed of light.
Acceleration is not the only use of resonant cavities in a particle accelerator. The interaction between the charged particles and any resonant cavity through which they pass also provides a basis for using a resonant cavity as a diagnostic tool, for determining the size, shape and other characteristics of the beam as it passes along the beam line; and it is to this purpose that the present invention is directed.
An electrically conductive structure acts as a resonator, or oscillator, when it has appropriate capacitative and inductive elements electrically connected in series in a loop. A simple oscillator can consist of an inductor and a capacitor connected to one another so as to form closed electrical loop. Resonance of such a circuit consists of alternating accumulations of an electric field in the capacitor and a magnetic field in the inductor. The frequency at which such an oscillator resonates is determined by the inductance (L) of the inductor and the capacitance (C) of the capacitor. Such a circuit is known as an LC network and its resonant frequency is given by the formula:
  f  =      1          2      ⁢      π      ⁢              LC            
A conductive structure as simple as a hollow tube that is closed at both ends can act as a resonant oscillator, and such an oscillator is known as a resonant cavity. In the ideal case of a cylindrical tube closed at both ends by parallel end plates, the spaced apart, parallel end plates act as a capacitor and the cylindrical wall of the tube acts as a single-turn inductor. In such a structure the periodic accumulation, discharge, and reversal of an axially extending electrical field, which extends between the capacitative end plates, alternates 90 degrees out of phase with the accumulation, discharge and reversal of a circular magnetic field that is centered on and extends along a circular path around the axis of the cylindrical tube, and which is largely contained within the cylindrical walls of the tube. The full cycle of the reversing electrical and magnetic fields repeats at the resonant frequency of the cavity.
Electrical energy can be introduced into such a cavity in the form of an RF signal transmitted into the cavity through a waveguide, to thereby maintain the cavity in a continuously resonant mode by overcoming ordinary losses due to power dissipation in the LC circuit.
In a charged particle accelerator, beams of charged particles, typically electrons or protons, are formed and are accelerated along a beam path. As noted above, resonant cavities are used to accelerate the particles in such beams. In such accelerators an evacuated beam tube defines a beam line that extends axially through multiple, spaced resonant cavities that are positioned along the beam line. The charged particles are accelerated in bunches as they pass through the successive resonant cavities. Each cavity must be appropriately positioned along the beam path and its interaction with the charged particles must be appropriately timed and otherwise optimized in several respects to achieve effective acceleration of the charged particles.
In particular, at each cavity the periodic formation of the electrical field must be properly phased and timed so that both its direction and its maximum strength coincide with the arrival of a bunch of charged particles at the center of the cavity. Further, the axial length of the particle bunch must be short compared with the wavelength of the RF signal used to excite the cavity. Finally, the axial length of the cavity in the direction of the beam must be sufficiently short that the electrical field extends in the same direction during the entire time required for the particle bunch to pass through the cavity.
A continuing challenge in the design and operation of particle accelerators is the determination of the precise characteristics of the particle beam at various points along the beam path. Such characteristics as the beam current, the cross-sectional shape of the beam, and the position of the beam relative to the axis of the beam tube are all affected by multiple factors related to the physical characteristics of the particle source and the beam line, including its accelerating cavities, as well as the operating parameters of the accelerator.
The ability to accurately and precisely diagnose the characteristics of the particle beam at various points is necessary in order to make the operating adjustments that are in turn required to optimize the quality of the beam. For this purpose, diagnostic resonant cavities may be interposed in the beam line at various points. Diagnostic cavities resonate in a manner similar to the resonance of the accelerating cavities. However, in the case of a diagnostic cavity the charged particle beam passing through the cavity generates a signal which can be transmitted out of the cavity through an appropriate waveguide. The nature and strength of this signal depend on the intensity, shape and position of the particle beam and thus can be used for diagnostic purposes.
Various techniques have been used to monitor the characteristics of a particle beam. See for example J. Ross et al., “Very High Resolution RF Cavity BPM” (beam position monitor), Proceedings of the 2003 Particle Accelerator Conference, p. 2545. A cavity intended as a beam position monitor is characterized by a voltage pattern which is, for example, positive in one side of the cavity and negative in the opposite side of the cavity. Such a cavity is useful for measuring the average displacement of the particle beam to one side of the cavity or the other.
As another example, a method of measuring the quadrupole moment of a beam with stripline beam position monitors for the purpose of determining the beam emittance was developed by Miller et al. (R. H. Miller, J. E. Clendenin, M. B. James, J. C. Sheppard, Proc. 12th Int. Conf. On High Energy Acc. (Fermilab, Batavia, 1983), SLAC-PUB-3186). In a related method, Whittim and Kolomensky disclosed the concept of using a resonant cavity to measure the beam dipole, quadrupole and higher moments. (D. H. Whittum and Y. K Kolomensky, Rev. Sci. Instr. 70 (1999), p 2300.) The idea of using a resonant cavity to measure the beam quadrupole moment was further developed by Kim et al. (J. S. Kim, C. D. Nantista, R. H. Miller, A. W. Weidemann, “A Resonant Cavity Approach to Non-Invasive Pulse-to-Pulse Emittance Measurement,” submitted to Rev. Sci. Instr.) The use of a cavity mode to measure the beam quadrupole moment has a much better signal to noise ratio than either the stripline or button pickup techniques, and can be used to measure much smaller beam features. In a quadrupole mode, the cavity is split into four quadrants, such that the cavity voltage alternates between positive and negative between adjacent quadrants and the cavity voltage is proportional to x2−y2.
The quadrupole-mode cavity measures <x2−y2>=σx2−σy2+<x>2−<y>2, where the angle brackets (< >) indicate an average over the particle beam population. Nearby dipole cavities measuring <x> and <y> can be used to subtract the two rightmost terms from this expression in order to give a measurement of σx2−σy2, where σx and σy are the root mean square beam widths in the x and y directions, respectively. In the absence of beam coupling between the x and y phase spaces, an emittance measurement can be performed by measuring the quadrupole moment at six locations along the beamline interspersed along the beamline focusing elements. Also, another cavity can be tilted by 45 degrees to measure <xy>, which can be used to diagnose and correct coupling between the x and y beam dimensions.
Quadrupole-mode beam position monitor cavities typically generate a much weaker signal than dipole-mode beam position monitor cavities. In order to make accurate measurements of low-emittance, high-energy beams, the measurement cavity should be optimized as much as possible. One way to improve measurement sensitivity is to use a multi-cell standing-wave cavity, for example a 9-cell structure as disclosed by J. S. Kim et al. (J. S. Kim, R. H. Miller, C. D. Nantista, “Design of a Standing-Wave Multi-Cavity Beam-Monitor for Simultaneous Beam Position and Emittance Measurement,” Rev. Sci. Instr. 76, 1 (2005)). In the disclosure of Kim et al., the shunt impedance as a function of beam offsets x and y is approximately R≃800 (x2−y2)2 Ω, where x and y are in units of millimeters. We define the shunt impedance as R=V2/P, where V is the voltage gained by a relativistic particle crossing a cavity containing a reference mode, and P is the power dissipated in the cavity walls. For a high-current train of pulses such as is expected to be used in future collider designs, such a diagnostic can adequately resolve the quadrupole moment of a beam with σx=1 μm, and σy<<σx. In order to make an accurate measurement in this case, the beam should be relatively close to the cavity axis, within a few microns.
Multi-cell structures are, however, more difficult to fabricate and tune. In order to obtain adequate shunt impedance for the mode, the structure is typically designed to operate in the π-mode. However, improper cell-to-cell transverse alignment can couple power to all modes in the quadrupole band, with phase advance ranging from 0 to π. (N. Barov, J. S. Kim, A. W. Weidemann, R. H. Miller, C. D. Nantista, “High-Precision Resonant Cavity Beam Position, Emittance and Third-Moment Monitors,” Proc. of the 2005 Particle Accelerator Conference.) This power must eventually be filtered out, which is more difficult in the case of small inter-mode spacing.
A resonant cavity incorporating two conductive rods extending into the cavity has been disclosed as having an approximately 100-fold increase in shunt impedance and has been suggested as being useful primarily as a beam deflector, and incidentally as a potential dipole-mode beam position monitor. (C. Leemann and C. G. Yao, “A Highly Effective Deflecting Structure,” Proceedings of the 1990 Linac conference, p. 232.) However, beam deflection in any particular direction requires only a dipole-mode structure, and thus there is no suggestion in the disclosure of Leemann and Yao of applications of more complex cavities based on higher-order resonant modes. Moreover, when the cavity of Leeman and Yao is optimized to function as a high-frequency (>5 GHz) diagnostic cavity with a reasonably large beam tube diameter, the effect of the rods is greatly diminished. For example, an 8.6 GHz cavity with a 1 cm diameter beam tube and the two rods of Leeman and Yao produces only approximately 40% more output power than a comparable cavity without the rods. Consequently beam position monitors based on resonant cavities and designed for electron accelerators operating at higher frequencies have consisted of simple resonant cavities without the two conductive rods suggested by Leeman.
In this regard, many electron accelerators operate with very short electron bunches, on the order of 10 picoseconds or less. In order to maximize the cavity output signal of such an accelerator, the diagnostic cavity frequency should be as high as possible, yet while also maintaining the condition that the cavity field should not change appreciably during the time period of the electron bunch. This favors a cavity frequency of at least 5 GHz.
For these reasons the two-rod cavity design of Leeman and Yao has not found acceptance as a beam position monitor, and there is nothing in the Leeman and Yao disclosure to suggest that increasing the number of rods would improve the performance of the cavity as a diagnostic cavity.
Accordingly, it is the object and purpose of the present invention to provide a resonant cavity that is useful for measuring and diagnosing the characteristics of a charged particle beam produced in a charged particle accelerator.
More particularly, it is the object and purpose to provide an improved apparatus and method for measuring the cross-sectional shape and dimensions of a charged particle beam.